[Otter74]

I'm guessing perhaps you're dealing with narrow tires that are particularly difficult to mount. As suggested above, don't push with your fingers or thumbs - those muscles aren't very strong, and you're going to tire your hand out quickly. Delfate the tire completely, work the bead down into the valley of the rim, particularly on the side opposite where you are beginning to work it off or the last part you're trying to seat, and when you're getting the last part of the bead on use your hands to push and roll it over rather than your fingers. If you're at home, a little dish soap on the bead helps, too. And while it's fine (often necessary) to use levers to get the bead off the rim, you should never use it to get the bead on. It's a great way to pinch-flat the new tube. If you have a particularly difficult rim-tire combination, a VAR bead jack can help.